The present invention relates generally to multi-level outphasing power amplifiers, and more particularly to asymmetric multi-level, multi-branch outphasing power amplifiers which have improved power efficiency and which do not require switching between multiple power supply voltages.
A problem of the prior art is that the need for high data rates and efficient spectrum utilization in modem wireless communication systems results in high peak-to-average power ratios of modulated signals. This requires associated RF power amplifiers (PAs) to operate much of the time at greatly reduced output power levels, which is referred to as large “power back-off” operation. Traditional power amplifiers have much lower efficiency (i.e., power delivered by the amplifier to the load divided by total power consumed by the power amplifier) under large power back-off conditions than under peak output power conditions.
In handheld devices, low power efficiency of the RF power amplifiers results in shorter battery lifetimes, and in base station applications the low power efficiency of the RF power amplifiers may result in wasted power and high heat sinking costs.
Outphasing or linear amplification using nonlinear components (LINC techniques) produce amplitude modulation by combining the output of two power amplifiers that are driven by constant envelope phase modulated signals. Outphasing power amplifier configurations also have been reported in the past using switching power amplifiers such as class D and class-E amplifiers. See “Efficiency of Outphasing RF Power-Amplifier Systems”, by F. Raab, IEEE Transactions on Communications, Vol. 33, No. 10, pp. 1094-1099, October 1985; this article discloses efficiency trade-offs and efficiency analysis for outphasing amplifiers. Also see US Patent Application Publication 2013/00210376 published Aug. 15, 2013 by Hur et al., entitled “LINC Transmitter with Improved Efficiency”; also see the articles “HF Outphasing Transmitter Using Class-E Power Amplifiers” by Beltran et al., IEEE MTT-S International Microwave Symposium, pp. 757-760, June, 2009, and “CMOS Outphasing Class-D Amplifier with Chireix Combiner” by Hung et al., IEEE Microwave and Wireless Components Letters, Vol. 17, No. 8, pp. 619-621, August 2007.
Prior Art FIG. 1 generally indicates how the drive signals S1(t) and S2(t) on conductors 14A and 14B, respectively, are generated in a basic out phasing system. (The basic outphasing technique is described in the above mentioned article “Efficiency of Outphasing RF Power-Amplifier Systems” by F. Raab.) In FIG. 1, drive signals S1(t) and S2(t) in the half-circle 17 are generated by signal separation circuit 15 in response to the incoming amplitude and phase modulated signal S(t) represented by the vector 17C. Drive signals S1(t) and S2(t) are represented by vectors 17A and 17B, respectively, in the half-circle 17. Specifically, in the outphasing power amplifier, the incoming signal with amplitude and phase modulationS(t)=a(t)ejϕ(t) is decomposed into two constant envelope phase modulated signals S1(t) and S2(t) on conductors 14A and 14B, respectively, asS1,2(t)=Aej[ϕ(t)±θ(t)]whereθ(t)=cos−1[a(t)/(2A)]and the constant amplitude A is defined as the maximum of a(t)/2, where ϕ(t) is the phase of the original amplitude and phase modulated signal S(t). The constant amplitude envelopes of signals S1(t) and S2(t) allow the use of switched-mode implementations of power amplifiers 3A and 3B, respectively.
Prior Art FIG. 2 is similar to FIG. 1 in the article “A 2.4-GHz, 27-dBm Asymmetric Multilevel Outphasing Power Amplifier in 65-nm CMOS”, by Philip A. Godoy, et al., IEEE Journal of solid-state circuits, Vol. 47, No. 10, pp. 2372-2384, October 2012, which is entirely incorporated herein by reference. (Also see related U.S. Pat. No. 8,164,384 entitled “Asymmetric Multilevel Outphasing Architecture for RF Amplifiers” issued Apr. 24, 2012 to Joel L. Dawson et al., also entirely incorporated herein by reference.) The above Godoy, et al. article shows an example of an asymmetric multi-level outphasing (AMO) transmitter including an “in-phase” input signal I(t) and a “quadrature” input signal Q(t) input to an AMO signal decomposition circuit 2 producing a pair of output signals Φ1 and Φ2 coupled to inputs of a pair of phase modulator circuits 1A and 1B, respectively. The outputs of phase modulators 1A and 1B are connected by conductors 14A and 14B to inputs of two switching power amplifiers 3A and 3B, respectively. Phase modulator 1A generates a signal S1(t) on conductor 14A and phase modulator 1B generates a signal S2(t) on conductor 14B.
In this example, a switching circuit 5A operates to selectively couple four supply voltages Vsup1, Vsup2, Vsup3, and Vsup4 to the supply voltage terminal 4A of power amplifier 3A and a switching circuit 5B operates to selectively couple Vsup1, Vsup2, Vsup3, and Vsup4 to the supply voltage terminal 4B of power amplifier 3B. AMO signal decomposition circuit 2 generates the power supply selection (i.e., power supply modulation) control signals A1(t) and A2(t) to switches 5A and 5B.
Power amplifier 3A generates a drive signal S1(t)OUT on conductor 7A and provides it as an input to a combiner 10 (which can be an isolating or a non-isolating combiner). Similarly, power amplifier 3B generates a drive signal S2(t)OUT on conductor 7B and provides it as another input to combiner 10. An output SOUT(t) of combiner 10 is coupled by conductor 10A to antenna 10B, which provides a load impedance. The above mentioned Godoy article provides a comprehensive explanation of how the various input signals may be generated. (A related reference is the article “Asymmetric Multilevel Outphasing Transmitter using Class-E PAs with Discrete Pulse Width Modulation” by SungWon Chung et al., IEEE MTT-S International Microwave Symposium, pp. 264-267, 23-28 May 2010.
Known Asymmetric Multi-level Outphasing (AMO) techniques use multiple power supply levels (e.g., Vsup1, Vsup2, . . . etc. in Prior Art FIG. 2) selectable by power supply selection signals A1(t) and A2(t) to improve the power efficiency at the RF power amplifier at “back-off” power levels.
Unfortunately, generating the multiple power supply levels and switching among them is undesirably/unacceptably power consuming and also is relatively difficult to implement. Another approach is to vary RF carrier signal duration, but this also causes undesirably high power consumption and is difficult to control.
Various kinds of switching power amplifiers, e.g., class D and class-E power amplifiers, have been used to implement power amplifiers such as amplifiers 3A and 3B in Prior Art FIG. 2. Basic class-E switching power amplifiers consist of a capacitor shunting a switching transistor, a series tuned load network, and a choke inductor. See F. Raab, “Idealized operation of the class-E tuned power amplifier”, IEEE Transactions on Circuits and Systems, Vol. 24, No 12, pp. 725-735, December 1977.
In outphasing power amplifiers, the individual power amplifiers (such as class D or class-E power amplifiers) typically are switching amplifiers and therefore are very power-efficient. The power efficiency is defined as the output power delivered to the load divided by the total power supplied to the outphasing amplifier by the single or multiple supply voltages Vsup1, Vsup2, . . . etc. The amount of output power delivered to the load decreases as the phase shift θ between the two driving signals S1(t) and S2(t) increases. Increasing the phase difference θ between the driving signals S1(t) and S2(t) results in a reduction in power efficiency. When the outputs of two such power amplifiers are combined or added together, there may be substantial power loss in the combiner which reduces efficiency, and the efficiency decreases as the output/load power decreases.
There are two different combining techniques for the outputs of the switched power amplifiers of an outphasing power amplifier. One technique is using “isolated” power combiners and another technique is using “non-isolated” power combiners. Non-isolating combiners may be of various types, including Chireix combiners. In basic outphasing, a large phase difference between S1(t) and S2(t) (when low power is required at the load) results in power dissipation in the combiners. Consequently, power efficiency is reduced when power delivered to the load decreases. The technique disclosed in the above mentioned Godoy et. al reference attempts to improve power efficiency at various power back-off levels by using multiple power supplies. Although this increases power efficiency compared to that of basic outphasing, the Godoy et. al technique has other shortcomings, including the fact that generation of multiple power supply levels is difficult and costly, the switching results in losses that reduce power efficiency, and synchronization problem between power supply switching and RF signal paths results in signal nonlinearity.
Thus, there is an unmet need for a way to improve the power efficiency of multi-level outphasing power amplifiers while RF power amplifier therein are operating at low “back-off” power levels.
There also is an unmet need for a way to provide improved power efficiency and simplified implementation of multi-level outphasing power amplifiers operating at large “back-off” power levels without generating and switching among multiple power supply levels to provide operating supply voltage and power to the internal power amplifiers of the multi-level outphasing power amplifiers.
There also is an unmet need for an improved multi-level outphasing power amplifier having the combination of higher data rates, more efficient spectrum utilization, and higher power efficiency than prior multi-level outphasing power amplifiers.
There also is an unmet need for an improved multi-level outphasing power amplifier which avoids the linearity problems of prior asymmetric multi-level outphasing (AMO) power amplifiers due to switching among multiple supply voltages and synchronization problem between supply voltage selection signals and RF driving signals.
There also is an unmet need for an improved outphasing power amplifier having higher peak output power than has been economically achievable in outphasing power amplifiers.
There also is an unmet need for an improved multi-level outphasing power amplifier which improves battery life in handheld devices.